The prior art is replete with exhaust hood apparatuses, mountable above a cooking station, and which primary functions are to draw air containing cooking by-products from the immediate vicinity of the cooking station and to expel this contaminated air to an external environment through an exhaust duct.
Since part of the cooking by-products is formed by minute particles of grease that have a tendency to stick to the plenum chamber of the exhaust hood, many exhaust hood apparatuses include a system to wash the plenum chamber and the exhaust duct periodically.
These washing systems typically include a valve assembly provided between a hot water source and an outlet nozzle mounted in the plenum chamber. A timer is often used to create a wash cycle by opening the valve assembly for a predetermined period.
To conform with known safety rules, conventional exhaust hood apparatuses usually include a safety mechanism designed to prevent the propagation of fires through the exhaust ducts linking the exhaust hood to an external environment. This safety mechanism is intended to prevent the temperature of the exhaust duct to exceed a predetermined maximum level which is called the flash point. The flash point in a plenum chamber or an exhaust duct varies with the level of contamination of the walls of the plenum chamber and of the exhaust duct. Indeed, the flash point decreases with the increase of grease or other particles sticking to these walls.
It is therefore imperative that the temperature in the plenum chamber and in the exhaust ducts stays below the flash point to eliminate the risks of fire propagation through the exhaust ducts.
One common safety mechanism consists of a damper that is automatically closed when abnormally elevated temperatures are detected in or near the exhaust duct. For example, U.S. Pat. No. 4,784,114 issued on Nov. 15, 1988 to Muckler et al. describes a kitchen ventilating system including a damper that is closed by automatically initiating the operation of a motor when a predetermined heat level is detected by a temperature sensor. The ventilating system proposed by Muckler also includes a spray wash apparatus operated by a control circuit. The spray wash apparatus is activated when a predetermined heat level is detected by a temperature sensor. However, if the fire producing the heat level detected by the temperature sensor is not inside the enclosure of the ventilating apparatus, the water exiting the spray wash apparatus will not assist to extinguish it since the damper is closed and therefore prevents the water to exit the ventilating apparatus. Another disadvantage of the ventilation system of Muckler is the fact that the smoke generated by an eventual fire may not be exhausted since the damper is closed.
U.S. Pat. No. 4,085,735 issued on Apr. 25, 1978 to Kaufman et al. describes an air ventilation and washing system having automatically activated electrical and mechanical fire control apparatus selectively responsive to changes in temperature in an exhaust duct of the ventilation system. The washing system is mounted inside the exhaust duct and is automatically activated should a temperature sensor detect a heat level that is above a predetermined threshold.
The system proposed by Kaufman is designed to extinguish the uncontrolled fire, not to cool the exhaust duct. Indeed, the water supplied to the washing system is hot, decreasing its efficiency to cool the exhaust duct.
Another disadvantage of the system proposed by Kaufman is that the washing system has conduits and water outlets along the entire length of the exhaust duct. However, most of the grease tends to accumulate in the plenum chamber, near the inlet of the exhaust duct. The water outlets away from the duct inlet are therefore not necessary for cleaning purposes. Furthermore, since ventilation systems are often mounted away from the external outlet of the exhaust duct, the cost involved in the installation of the washing system over the entire length of the duct increases significantly the total cost of the ventilation system.
Yet another disadvantage of the ventilation system of Kaufman is the fact that there is no provision to exhaust the smoke generated by an eventual fire.